The present invention relates to an abnormality determination for an intake air system of an internal-combustion engine (which will be hereinafter referred to as an “engine”).
Conventionally, a scheme for detecting a secondary air leak in an intake air system of an engine has been proposed. For example, Japanese Patent Application Publication No. 2003-148215 discloses controlling an intake air amount in an engine by adjusting an opening degree of a throttle valve. Occurrence of secondary air is determined based on the magnitude of a control correction term for the opening degree of the throttle valve. On the other hand, failure detection for a throttle valve mechanism and/or a sensor for measuring a pressure inside an intake manifold of an engine is conventionally implemented by detecting a broken wire, a short circuit and so on.
In an engine where an intake air amount is controlled by a variable valve actuation mechanism that changes a lift amount of an intake valve, the conventional secondary air leak detection scheme cannot be implemented. Further, it is difficult for the conventional failure detection scheme for a throttle valve mechanism and a sensor for measuring a pressure inside an intake manifold to detect a characteristic abnormality such as a deviation of a sensor measurement value from its initial state.
In order to solve these problems, the present invention aims at providing a scheme for determining an abnormality of an intake air system with higher accuracy in an engine where an intake air amount is controlled by adjusting a lift amount of an intake valve.